n. 1. One who, or that which, knocks; specifically, an instrument, or kind of hammer, fastened to a door, to be used in seeking for admittance. [ 1913 Webster ]

Shut, shut the door, good John ! fatigued, I said;
Tie up the knocker; say I'm sick, I'm dead. Pope. [ 1913 Webster ]

2. A person strikingly handsome, beautiful, or fine; one who wins admiration; a “stunner.” [ Slang. ] [ Webster 1913 Suppl. ]

3. A species of large cockroach, especially Blabera gigantea, of semitropical America, which is able to produce a loud knocking sound. [ Webster 1913 Suppl. ]

4. [ usually used in pl. ] a woman's breast. [ vulgar ] [ PJC ]